1. Field of the Invention
This invention relates generally to extruding teeth, such as gear teeth or spline teeth, and more particularly to use of multi-station tooling for extruding helical teeth in a workpiece, such as a gear blank.
2. Description of the Prior Art
Planetary gear units of the type used in automotive transmissions include ring gears having internal helical teeth rather than straight gears even though helical gear teeth are more difficult to form. The internal gear teeth must be formed with very precise dimensions and spacing in order to perform correctly.
Machining helical gears requires expensive tooling and equipment. Current processes for forming helical teeth include broaching, shaper cutting, grinding and hobbing, all being slow processes and requiring expensive capital equipment.
Back extrusion of internal splines or helical gears is much faster, but sometimes cannot produce in one operation a complex involute tooth profile to the required dimensional accuracy. Conventional back extrusion techniques produce tooth profiles that are accurate over most of the tooth shape, but are not within specification in certain regions due to difficult geometry or failure to fill certain volumes with metal during the forming process.
A source of noise produced by transmission gearing is produced by tooth-to-tooth interactions among gears during service operation. Noise sources have been shown to result from out-of-round condition of ring gears and from surface characteristics of the mating gear teeth.
Gears having similar surface finishes can produce vibration that results in audible noise. To reduce noise, gear makers strive to achieve tooth surfaces that are very smooth and precise. Cost of gear tooth finishing has increased as efforts to reduce audible gear noise have accelerated. Gear finishing processes, such as hobbing, broaching and grinding, all produce gears with tooth surfaces that are characteristic of the process and are very similar tooth-to-tooth.
A need exists in the metal forming industry for a method that forms gear teeth at high productivity rates having a completely formed tooth profile that meets a precise accuracy standard for dimensions and tolerances and produces a random and irregular surface finish on each tooth that is dissimilar to the surface finish of the mating teeth.